In scuba diving, it is very important to avoid conditions such as the "bends", also variously known as "Cassion's Disease", "compressed air illness" and "diver's palsy", a condition that occurs in individuals who, having been exposed to increased environmental pressure, are subjected to too abrupt a reduction in the pressure. To prevent such a condition, it is necessary for a diver to undergo gradual compression and decompression. The cause of this disorder is the rapid liberation of nitrogen from the fluids and tissues of the body as a result of the diver going too rapidly from high to low pressure. The presence of this nitrogen is due to the increase in solubility of the gas under higher pressure, as encountered in a cassion. As pressure is reduced, liberated gas forms bubbles in nerve tissue, muscles, brain and other body tissue. The only treatment for the bends is recompression and slow decompression. If a diver has risen too rapidly, he is placed as quickly as possible into an airlock and the air pressure is increased until the air bubbles in his body go into solution. The pressure is then very gradually lowered until normal air pressure is realized. In view of the severity of this condition, it is desirable to prevent its occurrence under normal diving conditions. For example, the U.S. Navy has derived a set of decompression tables that specify rates such as 60 feet per minute as an upper limit on the speed with which either an ascent or descent is undertaken.
Once in the water, a diver can become confused as to whether he is descending or ascending, especially in the dark or in murky water. This can cause problems since the diver can make a change in depth that is too rapid or he can dive to a dangerous depth. To aid the diver, apparatus has been developed to indicate to the diver both his depth and his rates of ascent or descent. In addition, some of these devices have included rate warning devices. For example, U.S. Pat. No. 3,992,948 discloses a system that outputs a visual or audible warning whenever a predetermined rate of ascent or descent is exceeded. In addition, the system indicates with a visual or audible signal the fact that the diver is ascending or alternatively descending.
In certain situations, such as cave diving, it is important that the diver have a continual indication of his depth and whether he is ascending or descending. The use of visual indicators can be distracting since a diver often concentrates intently on the task at hand. Visual indicators are therefore undesirable in this regard. In view of these problems, there exists a need for an indicating device that supplies continual information to the diver regarding the status of his dive and further, provides requisite warning signals for too rapid of an ascent or descent and too deep of a dive.